A new optical sensor for carbon dioxide has been developed based on the principal that the pH of a bicarbonate solution, which depends on the partial pressure of carbon dioxide with which it is in equilibrium, can be measured optically. Previous work had shown that it was possible to construct probes using two plastic optic fibers to measure the light transmission of a pH indicator dye solution contained inside a sealed silicone rubber tube at the end of a 60 cm. probe. The transmission signal is proportional to the partial pressure of CO2 with good sensitivity and linearity over the range of 20 to 80 mm. Hg. Current work has been connected with the evaluation of pH indicating dyes other than phenol red which has been used previously and the extension of the two fiber approach to a single fiber system suitable for investigating very localized PCO2 gradients. In this case, the sensor uses a fluorescent pH indicator, carboxy fluorescein, contained in the tip of a glass micropipet. A single glass fiber is sealed into the pipet to form the rear seal of the sensing volume. A plug of silicone rubber keeps the sensing solution in the pipet and allows CO2 to come to diffusional equilibrium with the solution. Probes made to date show response times on the order of 2 min. and a projected sensitivity of about 2 mm. Hg.